Exhaust gas temperatures (EGT) of gas turbine engines are often sensed with pyrometers to provide engine control feedback. Pyrometers operate on the basis of sensing the quantity or intensity of radiant energy emitted proportional to the fourth power of absolute temperature. Because the lens of a pyrometer used to sense EGT is typically located a distance from the area of focus, the lens is often enclosed within a tubular-shaped structure having an open end directed at the target region where temperature is to be sensed. The pyrometer itself is located in the turbine section, and is therefore subject to oxidation and extremely high temperatures. For this reason, the tubular-shaped structure is typically a superalloy or stainless steel. The lens is typically formed from such materials as sapphire (alumina) and silica glass.
In the high-temperature oxidizing environment of a gas turbine engine, the precision of the pyrometer can be affected by deposits on the lens, an event referred to as "clouding." For example, clouding of a pyrometer's lens can cause the pyrometer to indicate an EGT that is lower than what actually exists in the turbine section, causing the engine fuel control system to increase the EGT and result in flow path hardware such as blades, vanes and shrouds operating at higher temperatures than intended. A primary deposit on the lenses of certain EGT pyrometers has been determined to be chromium-rich from an unknown source. It would be desirable if the deposition mechanism for chromium and other potential deposits on lenses used at high temperatures could be prevented.